1. Technical Field
The present invention relates generally to an apparatus and method for liquefied natural gas (LNG) carrier propulsion, and more particularly to an apparatus and method for propulsion of an LNG carrier that comprises a boil-off gas re-liquefaction apparatus and a diesel engine propulsion system.
2. Description of the Related Art
Generally, liquefied natural gas (LNG) is produced by cooling natural gas into a cryogenic liquid state at a production terminal and transported by LNG carriers to distant locations where it is to be regasified.
Since the liquefaction of natural gas occurs at a cryogenic temperature of approximately −163 degrees Celsius at ambient pressure, LNG is likely to be vaporized if the temperature of LNG increases above −163 degrees Celsius at ambient pressure. Although an LNG storage tank for the LNG carrier is equipped with thermal insulation structure, it is impossible to completely prevent heat ingress to LNG through the storage tank, and as a result, LNG is consistently vaporized and boil-off gas is generated in the LNG storage tank during LNG transportation.
A conventional LNG carrier employs a steam turbine propulsion system, which is driven by steam obtained by burning boil-off gas in a boiler. However, such a steam turbine propulsion system has a drawback of low propulsion efficiency.
On the other hand, in the case where boil-off gas is not used as fuel for propulsion of an LNG carrier, it is possible to increase the economic values of the boil-off gas by recycling it back to the storage tank. For this purpose, diesel engine propulsion with a re-liquefaction system has been developed, which has a boil-off gas re-liquefaction apparatus installed in the LNG carrier to return boil-off gas back to the storage tank and includes a diesel engine having high propulsion efficiency as the main propulsion source.
Especially for LNG carriers, to maintain the high reliability, the LNG carriers are required to have countermeasures against emergencies, such as the malfunction of main diesel engines in the diesel engine propulsion system. Furthermore, since it is necessary to install two main diesel engines in the prior art, the diesel engine propulsion system has problems in that the construction of an LNG carrier is complicated and initial and maintenance costs are increased.